Various solutions to the problem of limiting operator access to telephone conversations have been proposed. One solution involves automatically cutting-off an operator after a prescribed time interval sufficient in duration for the operator to perform a legitimate monitoring operation. Another solution involves the scrambling, or distorting, of speech signals enroute to a monitoring operator so that the speech content cannot conveniently be ascertained.
The solution of automatically cutting-off an operator after a prescribed time interval is illustrated in U.S. Pat. No. 3,544,735, which issued to E. Olszewski et al on Dec. 1, 1970. This patent discloses a service observing system in which a pair of timers control the speech signal cut-off to an observing operator and other apparatus selectively recycles the timers to delay the automatic cut-off under specific conditions. Although effective for certain purposes, this solution has the disadvantage of allowing portions of a conversation to be overheard.
For verifying the busy condition of telephone lines, one telephone company has used a speech scrambling circuit interposed between a line being verified and an operator. The scrambling circuit distorts a speech signal by reversing its polarity at a 1400 Hz rate. Analog signal transmission gates inserted in each side of the telephone line and similar gates cross-connecting the two conductors of the line are alternately controlled to switch the speech signal polarity on the conductors at the 1400 Hz rate. The effect is said to render the speech signal unintelligible.
It is also known to distort an electrical speech signal by clipping (amplitude limiting) and rectifying.
In other related art fields involving speech conditioning and distortion, U.S. Pat. No. 3,647,970, which issued to G. P. Flanagan on Mar. 7, 1972, discloses a circuit for simplifying speech waveforms. A speech signal is converted to another signal having substantially constant upper and lower levels with abrupt transitions from one level to the other, the abrupt transitions being in time correspondence to amplitude changes in the speech waveform that exceed a predetermined rate of change. The circuit includes a high-pass audio frequency filter and clipping means for converting a filtered waveform to the constant level square wave. With reference to FIG. 4 of the patent, the simplified square waveform is scrambled by differentiating it to produce voltage spikes at the pulse transitions. The spikes are rectified and the rectified signal used to trigger a single-shot multivibrator. Each pulse output of the multivibrator is converted to a series of pulses which are transmitted to a receiving site where the scrambling technique is reversed to recover the original simplified square wave.
U.S. Pat. No. 3,614,323, which issued to W. L. Roberts et al on Oct. 19, 1971, discloses a circuit for use on subscriber loop carrier circuits to prevent eavesdropping of a voice frequency subscriber by a carrier frequency subscriber on the same loop. The circuit is composed of a rectifier section in series with a low-pass filter section to allow the transmission of DC and low frequency control signals while blocking voice frequency signals. A high-pass filter section bridges the rectifier and low-pass filter sections to allow the transmission of high-frequency carrier signals.
The known speech conditioning and scrambling circuits mentioned above appear to suffer in varying degree from one or more of the following deficiencies: some allow the scrambled speech signal to be decodable, others incompletely destroy the intelligibility of the speech content, while others destroy the recognizability of the signal as speech.